Metallic aluminum is produced industrially by igneous electrolysis, namely by electrolysis of alumina in solution in a molten cryolite bath, known as an electrolysis bath, using the well-known Hall-Héroult process. The electrolysis bath is contained in cells which comprise a steel container coated on the inside with refractory and/or insulating materials, and cathodic elements located at the bottom of the cell. Anode blocks made of carbonaceous material are partially immersed in the electrolysis bath. Each tank and the corresponding anodes form what is often called an electrolysis cell. The electrolysis current, which circulates in the electrolysis bath, and possibly a layer of liquid aluminum via the anodes and the cathodic elements, causes the reduction reactions of alumina and also makes it possible to maintain the electrolysis bath at a temperature of about 950° C. by Joule effect.
French patent application FR 2.806.742 (corresponding to American U.S. Pat. No. 6,409,894) describes installations in an electrolysis plant designed for the production of aluminum.
According to the most widespread technology, the electrolysis cells comprise a plurality of anodes said to be “pre-baked”, made of carbonaceous material. These are consumed during the aluminum electrolytic reduction reactions.
Gases, especially carbon dioxide, are generated during the electrolysis reactions and naturally accumulate in the form of gas bubbles under the generally substantially flat and horizontal lower surface of the anode, which influences the overall stability of the cell.
The accumulation of these gas bubbles causes:                electrical variations and instabilities,        a high frequency and long duration of anode effects,        an increased possibility of the opposite reaction and therefore a loss of productivity because of the short distance between the layer of aluminum produced and the CO2 bubbles,        an increased consumption of carbon and the formation of harmful gases because of the transformation of CO2 as it comes into contact with the carbon.        
The use of pre-baked anodes with carbonaceous anode blocks comprising one or more grooves in their lower part is known; these facilitate the removal of the gas bubbles and prevent them from building up in order to solve the problems stated above and to reduce energy consumption, as shown in Light Metals 2005 “Energy saving in Hindalco's Aluminum Smelter”, S. C. Tandon & R. N. Prasad. The grooves make it possible to decrease the average free path of the gas bubbles under the anode to get out from the space between the electrodes and thereby to reduce the size of the bubbles which are formed under the anode.
The value of the use of grooves has already been studied and proven, for example in Light metals 2007 p. 305-310 “The impact of slots on reduction cell individual anode current variation”, Geoff Bearne, Dereck Gadd, Simon Lix, or Light metals 2007 p. 299-304 “Development and deployment of slotted anode technology at Alcoa”, Xiangwen Wang et al.
It is also known, from the following documents:    WO 2006/137739, to use finer grooves (about 2 to 8 mm) than those commonly used (about 8 to 20 mm) so as to optimize the useful carbonaceous mass and the exchange surface;    U.S. Pat. No. 7,179,353, to use an anode block comprising grooves leading to a single side or side surface of the anode block, and more particularly towards the center of the electrolysis cell so as to improve alumina dissolution.
A well-known limit to the use of these grooves results from the fact that the depth of the grooves from the lower surface of the anode blocks is limited in order not to disturb the mechanical and physical intactness of the carbonaceous anode blocks. However the carbonaceous anode blocks are gradually consumed during the electrolysis reaction over a height greater than the depth of the grooves so that the duration of the grooves of an anode is shorter than the lifespan of the anode. Consequently, for a certain amount of time during the lifespan of the anodes the lower part of the anode blocks no longer has any groove. The problems stated above for anodes without grooves then become noticeable.
As stated in Light metals 2007 p. 299-304 “Development and deployment of slotted anode technology at Alcoa”, the depth of the grooves is limited for reasons of intactness mainly in the case of grooves formed by molding on crude anode blocks so that the beneficial effects resulting from the presence of the grooves can be observed only during part of the lifespan of the anodes. The grooves create weaknesses in the crude anode blocks which then split during transport, storage or baking.
In practice it also proves difficult and expensive to reliably obtain by sawing baked anode blocks anodes with grooves as deep as the height of the anode block that will be consumed. The mechanical strains and vibrations exerted by sawing blades cause the carbon blocks to crumble, craze, and then burst. Anode sawing additionally proves to be an expensive exercise, particularly on account of the high cost of the sawing equipment, the large amount of energy required, and the collection and treatment of the powders produced by sawing.
The dimensions of the anode blocks for anodes commonly used are of about 1200 to 1700 mm in length, 500 to 1000 mm in width and 550 to 700 mm in height, with one to three grooves of a depth generally ranging between 150 and 350 mm.
For a 600 mm high anode block with a height of consumable carbon of 400 mm and a 250 mm deep groove, the groove produces a beneficial effect during only 62.5% of the lifespan of the anode.
A first aim of the invention is to propose another type of anode to solve the problems of removing the gas building up under the anodes without compromising the intactness of the anode blocks while they are being manufactured, stored, transported or used.
Another aim of the invention is to propose anodes making it possible to cure the disadvantages stated above, i.e. to propose anodes producing a beneficial effect for a greater length of time without compromising the intactness of the anode blocks while they are being manufactured, stored, transported or used.